1. Field of the Invention
The present invention relates to an encoder attached to an electric motor or an apparatus driven by the electric motor, in particular, to an encoder having durability against vibration or shock.
2. Description of the Related Art
In recent years, an encoder, attached to an electric motor or an apparatus driven by an electric motor, is often used for equipment such as a press machine, wherein excess vibration or shock may be applied to equipment. Since the encoder includes many electronic parts, excess vibration or shock may cause a malfunction or damage the encoder. Therefore, it is desired to improve durability of the encoder by reducing vibration or shock applied to the encoder.
Generally, an encoder has a rotating code plate configured to rotate in synchronization with a detected object such as an electric motor shaft, and a printed board having a detection circuit configured to detect pattern information or the like of the rotating code plate. Since the printed board is positioned above the rotating code plate in many cases, several peripheral portions of the printed board are fixed to a housing by means of a screw or adhesive. In this regard, a technique for improving the rigidity of the housing has been proposed. For example, Japanese Unexamined Patent Publication (Kokai) No. 2008-3064 discloses a technique for improving the rigidity of a board fixing member 7 by gradually increasing an inner diameter of fixing member 7 toward a board 6.
However, when excess vibration or shock is applied to the encoder, a center portion of the printed board may be widely vibrated, and a malfunction or breakage may occur in electric parts or the like mounted to the center portion. As such, in order to improve the durability of the encoder against vibration or shock, it is desired to reduce the vibration of the center portion of the printed board. As a technique for solving the problem, Japanese Unexamined Patent Publication (Kokai) No. 2004-325171 discloses an acceleration sensor arranged in an encoder wherein shock is reduced by accelerating the encoder in the inverse direction, Japanese Unexamined Patent Publication (Kokai) No. 7-243869 discloses a technique for fixing an encoder via a vibration-proof spring member (or a leaf spring), and Japanese Unexamined Patent Publication (Kokai) No. 2003-287025 discloses a technique for using resin to harden a portion, vibration resistance of which should be improved.
Further, in Japanese Unexamined Patent Publication (Kokai) No. 2010-286452, a print board 5 is supported by a board supporting piece 9 made from a vibration proof material, such as rubber, in order to reduce the vibration.
In the method of Japanese Unexamined Patent Publication (Kokai) No. 2004-325171 or Japanese Unexamined Patent Publication (Kokai) No. 7-243869, it is necessary to arrange the acceleration sensor, a complicated processing circuit or the leaf spring in the encoder, and thus the encoder becomes expensive and/or difficult to be made compact. Further, in the method of Japanese Unexamined Patent Publication (Kokai) No. 2003-287025, it requires a huge amount of effort and cost to reinforce the portion of the encoder by resin, and thus the cost of the encoder may be increased and/or production efficiency of the encoder may be decreased.